The new Harrison Ford flick that’s coming out this Friday.
Why do they use the Gladiator music in the trailer?
Hmmmmmmmmmm???
Maybe the same reason they seem to use music from The Omen in every scary movie trailer. It’s too tell the audience, “oooh, this movie’s scary!!”, just in case they don’t get it from the trailer. And it sounds cool.
Oh!
Well, it just sounds very Roman, is all. 
It’s okay, in one of The Gladiator trailers, they used music from Conan The Barbarian!
Oh, that’s where I heard it before!
I’m guessing that the trailer was cut before they finished the final scoring of the film, so they used something they had available. I’ve noticed the theme of Edward Scissorhands being used on some other trailers a couple of years ago.
Did anybody else think this was a spoof trailer for the new Austin Powers movie when they first saw it?
They also use the theme for Crimson Tide in some of the trailers. Presumably to also tell you its a submarine movie.
Its fairly common to use the theme songs from one movie in the trailer of another in order to achieve a desired emotional response that you might not get from brand new music.
Commonly used movie theme music used in trailers:
Stargate - Fun action adventure films
Waterworld - Swashbuckling adventure films
Rudy - Stiring epics
Aliens - Intense action dramas
Gladiator - Intense action war dramas
Crimson Tide - Intense action war dramas
Conan - Midevil action movies
It’s music that’s generally recognizable enough to be familiar while being obscure enough that most people aren’t like “that’s the Star Wars theme!”.
And some commonly used classical music
Carmina Burana (often confused with Conan music)
Bethovan’s Ode to Joy (Die Hard x-mas music)